This invention relates to an apparatus and method, for performing either photorefractive keratectomy (PRK) or phototherapeutic keratectomy (PTK) on the anterior surface of the cornea of the eye using a feedback-controlled segmented laser beam.
The now widely recognized property of the excimer laser operating at a wavelength of 193 nm to ablate corneal stromal tissue has given rise to a number of inventions for controlling this radiation in a manner to reshape the full optically effective anterior surface of the cornea. These inventions include: en face methods where the depth of ablation is proportional to the intensity of radiation and duration of application; and tangential methods where ablation depth is a primarily a function of the axial position of the cornea relative to the region of laser radiation. Despite significant differences among the various methods and their apparatuses, they all lack the capability of being able to rapidly measure the topography of the cornea and based on these measurements modify the laser radiation pattern continually throughout the photoablation process. Contained in prior art are many claims for apparatuses and methods for controlling the ablation process through computer conducted feedback control--e.g. U.S. Pat. Nos. 4,941,093, 4,724,522, 4,729,372. Primarily two problems militate against the effectiveness of these techniques. The first is in the limitation of the keratometry techniques used to measure surface topography of the cornea because of the diffuse surface of the cornea undergoing ablation; the second limitation lies in the inability of these techniques to control the laser beam cross sectional intensity profile in response to instant by instant changes in the corneal topography. Means for surmounting these limitations are envisioned by the present invention.
Present state-of-the-art excimer lasers achieve beam cross sectional intensity uniformity to within about 5%; additionally, en face apparatuses deliver the radiation in the form of expanding or contracting circular or elliptical projections that are very accurately calculated and controlled to achieve desired ablation depths. Notwithstanding these controls, the standard deviation from desired for most PRK test groups with less than 5 D (diopters) of preoperative myopia is 1 D, implying a deviation of some 1/5=20% from the desired result which is some 4 times higher than the inherent accuracy (5%) of the procedure. Further, scanning electron microscope photographs of ablated corneas evidence surface irregularities some 8 to 10 times larger than preoperative corneas. These results suggest an inherent nonlinearity in the ablation of the corneal tissue and that regardless of how uniform the radiation reaching the cornea is, the ablated cornea will always depart from the nominal desired post operative goal.
Existing techniques for topographical measurement (keratometry) of the full anterior surface of the cornea are based upon either reflection or projection of reference light patterns. Because the removal of the epithelium from the cornea prior to PRK results in exposing the stromal corneal surface which is not optically reflective, keratometry techniques based upon reflection are unsuitable. However, research has found that projective techniques utilizing rasterstereographic imaging such as the
Technologies Corneal Topography System (U.S. Pat. No. 4,995,716) has successfully been used on deepithelialized and freshly keratectomized corneas. The PAR CTS projects a grid on the cornea which is then imaged by a video camera, digitized and analyzed to produce a tabulation of corneal elevations versus corneal diameters by means of algorithms that require about a half minute to compute by a 486 50 Mhz equivalent computer.
Although the PAR CTS can measure corneal surfaces to within an accuracy of about 0.1 diopter which is within the limit for achieving complete refractive correction of the human eye, it can be useful in terms of the present invention only if it can be made to operate in conjunction with an apparatus that modulates the cross sectional intensity of the laser beam so that once a measurement is made on a region of the cornea, it can be compared with a desired calculated value and the intensity of the laser corresponding to the said region be changed to achieve the desired value. Such a procedure, to be successful, must make measurements and supply the corrective action to a modulated laser beam many times over the duration of the ablation process--perhaps as often as every laser pulse.
Recent developments in the field of three-dimensional integrated micromechanical structures (U.S. Pat. No. 4,918,032) and in constructing leaf spring switches (U.S. Pat. No. 4,681,403), and in fabricating bending joints (U.S. Pat. No. 4,953,834) enable a means for modulating a light beam. Existing modulators are limited in regard to application to the present invention in that they are either based upon reflection (rather than transmission) or are limited in efficiency. It is therefore one object of this invention to provide a micromechanical structure that functions by modulating transmitted laser radiation and doing so efficiently.
The recent advent of high speed low cost computers along with high speed analog to digital conversion devices afford a means for implementing a system for photoablative topographical control that until recently, would have been either too expensive and/or physically impossible. Accordingly, it an object of this invention to implement such a system using the available technology.